The present invention generally relates to controls for induction machines, and more specifically, to controls for brushless doubly-fed induction machines, including both motors and generators.
Doubly-fed induction machines have been used as variable speed electric motors or generators. Generators of this type have been controlled with a power converter that has a lower power rating than the machine electrical power output, while motors of this type have been controlled with a power converter having a lower electrical power output than the motor mechanical power output. The prior art also teaches that wound rotor induction machines having a stator connected to an alternating current (AC) power line can be controlled with a field oriented or flux vector control that is connected to the rotor to provide accurate control of the machine currents and torque when the machine is used as either a generator or a motor. FIG. 1A is a power circuit block diagram illustrating this prior art configuration. The stator of a doubly-fed machine 10 is connected through current sensors 20 to an AC power line 14, which also supplies power to a current regulating motor control 12. Control of the current in rotor leads 22 controls the torque. The mathematical model and design basis for such a control, including the use of rotor position and stator and rotor currents to determine the position of the flux vector, are disclosed in Chapter 13.1 of the textbook xe2x80x9cControl of Electrical Drives,xe2x80x9d by Werner Leonhard, Springer-Verlag (1985).
Flux vector control provides substantially independent control of the distribution of excitation current between the rotor and stator, and of the quadrature stator current, which determines torque. The applied AC stator voltage and machine characteristics determine the total excitation current. The control regulates the stator portion of excitation in response to a reactive current reference and commands the necessary rotor excitation current to attain the required total excitation of the machine. This type of control accurately regulates the excitation and quadrature (torque producing) stator currents within preset limits and provides accurate torque control within preset limits, even if external loads exceed the rated machine or control capability.
As taught by the above-noted Leonhard text and other references, wound rotor machines that have a stator connected to the AC power line require power to flow from the rotor connection to the control when motoring at sub-synchronous speeds, which are speeds below the synchronous speed at which the frequency of the power at the control connection to the rotor of the machine is zero Hz. When the machine is operating as a generator, power flows into the rotor at subsynchronous speeds and from the rotor to the control at super-synchronous speeds.
Flux vector control of singly-fed induction machines, i.e., of a conventional AC induction motor 30, is also taught by the Leonhard textbook and this technique is commonly used in industrial motor and generator controls. Controls 26 all rely on position feedback 32 of rotor position, or electrical measurements of the stator, to provide the information needed to estimate the flux in the rotor. FIG. 1B is a power circuit block diagram of this prior art control configuration.
Control of the rotor with an inverter 44 in a doubly-fed wound rotor generator 36 for standalone applications is shown in the power circuit block diagram of FIG. 1C. Typically a DC bus power source 42 supplies control power to inverter 44 until the power output of generator 36 to inverter inputs 34 is adequate to supply control power. Inverter 44 controls the frequency and voltage of generator 36 rotor inputs 46. Voltage taps 18 are monitored for control of output voltage. This configuration is also taught by Leonhard and by other prior art references.
The slip rings of wound rotor doubly-fed machines can be eliminated with brushless doubly-fed machines of several types. These include dual rotor-stator induction machines (referred to below as xe2x80x9cType 1xe2x80x9d), such as disclosed in U.S. Pat. Nos. 3,183,431; 3,571,693; 4,229,689; 4,246,531; 4,305,001; 4,472,673; 4,701,691; 5,886,445; and 6,278,211. Single rotor-stator induction machines with two sets of stator windings of different pole counts (referred to below as xe2x80x9cType 2xe2x80x9d) are disclosed in U.S. Pat. Nos. 3,183,431; 5,028,804; and 5,239,251; and in other references listed therein. Reluctance machines (referred to below as xe2x80x9cType 3xe2x80x9d) are disclosed in U.S. Pat. No. 5,359,272 and by Xu et al. in xe2x80x9cA Novel Wind-Power Generating System Using Field Orientation Controlled Doubly-Excited Brushless Reluctance Machine,xe2x80x9d IEEE, pp. 408-413 (January 1992). Brushless doubly-fed induction machines of Type 1 with reverse phase rotor connections, and of Type 2, have a rotor construction that tightly magnetically links the two stator winding sets through the rotor currents, so that the total number of poles is equal to the sum of the number of poles of the two stator winding sets. When they are synchronously controlled, their speed is proportional to the sum of the two stator frequencies, and the torques on the shaft from the two sets of stator currents are additive.
Brushless doubly-fed induction machines with one stator connected to the AC power line also require power to flow from the other doubly-fed connection, i.e., the other stator, to the control when motoring at sub-synchronous speeds or generating at super-synchronous speeds. The synchronous speed in revolutions per second, at which the frequency of power at the control connection to the stator is zero Hz, is equal to the quotient of twice the AC power line frequency divided by the sum of the number of poles of the two stator windings. Several of the above-noted references also teach that there is a discontinuity in the control of these machines at the speed above synchronous speed where the rotor frequency is equal to zero Hz. No power can be transferred between the stators by the machine when the rotor frequency is zero. The speed, in revolutions per second, at which this discontinuity occurs is equal to twice the AC power line frequency divided by the number of poles in the stator connected to the AC power line. Thus, the speed range over which a brushless doubly-fed induction machine can be smoothly controlled is from zero speed through the synchronous speed, and up to nearly the discontinuity speed, where the rotor frequency is zero.
The flux vector control techniques developed by Leonhard and others for wound rotor machines have been shown to apply also to brushless doubly-fed induction machines. Papers describing these adaptations include: (1) D. Zhou et al., xe2x80x9cField Oriented Control Development for Brushless Doubly-Fed Machines,xe2x80x9d Proceedings of IEEE IAS Annual Meeting, San Diego (1996); (2) Xie Lun et al., xe2x80x9cThe Research of Brushless Doubly-Fed AC Excited Induction Motor Drive,xe2x80x9d Proceedings of Fifth International Conference on Electrical Machines and Systems (2001); and (3) B. Hopfensperger et al., xe2x80x9cCombined Magnetizing Flux Oriented Control of the Cascaded Doubly-Fed Induction Machine,xe2x80x9d IEEE Proceedings on Electric Power Apparatus (July 2001). The foregoing references teach flux vector control of singly fed induction machines, doubly-fed wound rotor induction machines and brushless doubly-fed induction machines and inverter control of standalone generators like that shown in FIG. 1C. However, none of these prior art references discloses or suggests a method for control of brushless doubly-fed induction machines that achieves specific desirable operating capabilities for such a machine. It would clearly be desirable to eliminate the position sensor typically used in the prior art and determine rotor position from electrical variables. It would also be desirable to develop a method of substantially xe2x80x9cbumplessxe2x80x9d doubly-fed motor connection of such a machine to an AC line at or near the zero Hz speed.
There are certain advantages to operating a brushless doubly-fed induction machine with one stator shorted, which are not disclosed in the prior art. For example, it would be desirable to employ an inverter or flux vector control of one stator of a doubly-fed motor, with the other stator shorted, at sub-synchronous speeds. It would also be desirable to provide flux vector control of one stator of a doubly-fed motor, with the other stator shorted, and then measure the current in the shorted stator to determine the shaft speed and torque. Furthermore, it would be desirable to develop a method for substantially bumpless switching between a shorted stator sub-synchronous motor operating mode and a higher speed field oriented mode of operation with the stator that was previously shorted connected to the AC line. It would also be desirable to develop a method of maintaining a near-constant motor power factor at all operating loads applied to a brushless doubly-fed induction machine.
It would be desirable to develop a method of controlling a brushless doubly-fed induction machine to operate as either a standalone generator or as an AC line-connected generator. It would also be desirable to control the speed of an engine-driven generator as a function of load to operate the engine at the lowest speed that provides adequate torque margin for short-term overloads
In accord with the present invention, an electronic power converter configured as a three-phase current regulator supplies current to the first stator of a brushless doubly-fed induction machine with the second stator open, shorted, connected to an AC line, or connected to a load. Control of the current vector into the first stator controls the torque of the machine when used either for motoring or generating power with the second stator connected to an AC line. The electronic power converter is configured as an inverter when the machine operates as an independent generator that is not connected to the AC line.
A processor is programmed to operate the machine as a speed or torque controlled motor, from zero speed to a speed that is greater than a synchronous speed of the machine. At the synchronous speed, the first stator input frequency is zero when the second stator is connected to the AC power line. A speed range from zero up to almost twice the synchronous speed with full torque can be attained at all speeds with a processor based controller that is rated for about half the motor output power at maximum speed. In addition, a speed range from zero to one and a half times the synchronous speed can be attained in variable torque applications, with torque proportional to the square of speed, using a processor-based based controller rated at about one-third the motor output power at maximum speed.
For variable speed drive applications requiring continuous torque at all speeds, a processor-based control 50 in accord with the present invention operates the machine 40 as a flux vector controlled doubly-fed motor, with AC switch 16 connected to AC power line 14 closed (see FIG. 2A). The flux vector control continuously maintains stator S1 flux orientation relative to the applied AC power line voltage within the capability of the controller, even for suddenly applied loads or loads beyond the torque capacity of the motor. Full torque capability is provided by the control at all speeds from zero to maximum speed, with no switching of modes after the initial AC line connection.
FIGS. 3A, 3B, and 3C are plots 76 and 78 of power, a plot 80 of stator S1 frequency and a plot 82 of voltagexe2x80x94all versus speed, for a typical dual 4-pole brushless doubly-fed induction motor 40 connected to 60 Hz AC power line 14. Both stator windings are identical, in this example, and processor-based control 50 is rated at about 50% of the motor rating at maximum speed. The synchronous speed is 900 RPM, and the rated maximum speed is about 1750 RPM, in this example. Operation is started at zero speed by controlling the S1 excitation with AC switch 16 open to synchronize the generated S2 voltage on taps 18 with AC power line 14, then automatically closing AC switch 16 to accomplish bumpless power application to the motor. Control 50 is then automatically switched to a doubly-fed flux vector motor control mode for operation over the speed range of zero to about full rated speed. AC power line 14 supplies power 78 proportional to output torque to stator S2 at all speeds. Neglecting losses, this power ranges from zero at no load to about 50% of the rated motor output power at rated torque. Again neglecting losses, the controller absorbs power 76 proportional to the product of torque and the difference between the synchronous speed and the operating speed from stator S1 at speeds below the synchronous speed. This power absorption ranges from 50% of motor rating at rated torque and zero speed to zero at synchronous speed. The controller supplies power 76 proportional to the difference between the synchronous speed and the operating speed to S1, at speeds above synchronous speed. This power ranges from zero at synchronous speed to almost 50% of motor rating at about the full rated speed and rated load. Control 50 output frequency 80 ranges from 60 Hz in the rotation direction opposite to that of AC power line 14 at zero speed, to zero at half speed, and almost 60 Hz in the AC line rotation direction at maximum speed of about 1750 RPM. Control 50 output voltage 82 ranges from 100% voltage output at zero speed to zero at half speed, and to 100% output at maximum speed. Electronically reversing the phase of the S1 input and reversing the phase of the AC line voltage applied to S2 prior to synchronization reverses the direction of rotation of the machine.
For constant torque variable speed drive applications that can tolerate a momentary zero torque output when changing operating modes, the dual-mode control of the present invention provides a substantial cost reduction by eliminating the need for controller 50 to absorb power below synchronous speed. This capability is accomplished with the power circuit block diagram of FIG. 2B, where processor-based control 50 operates machine 60 with S2 shorted by a shorting switch 62 below synchronous speed and switches to flux vector controlled doubly-fed motor control mode with S2 connected to AC power line 14, for operation above synchronous speed. FIGS. 4A, 4B, and 4C shows plots 90 and 92 of power, a plot 96 of stator S1 frequency, and a plot 98 of voltagexe2x80x94all versus speed, for dual 4-pole brushless doubly-fed motor 60 connected to 60 Hz AC power line 14. The S1 and S2 windings are identical and controller 50 is rated at about 50% of the motor rating at maximum speed. Operation is started at or about zero speed by closing shorting switch 62 with AC switch 16 open (as indicated in a box 86). The S1 frequency shown in plot 96 and the voltage shown in plot 98 are both proportional to speed up to the half speed level, and the controller output shown in plot 92 is proportional to motor output power, reaching a maximum of 50% of rated motor power at synchronous speed and rated load, neglecting losses. At about synchronous speed, shorting switch 62 is opened, the S1 excitation is controlled by the processor-based control 50 to synchronize the generated S2 voltage at taps 18 with AC power line 14. AC switch 16 is then closed to accomplish bumpless AC line power application to the motor. Processor-based control 50 then switches to doubly-fed flux vector motor control mode, as indicated in the portion of the plot under a box 88, for operation over the range from synchronous speed to about full rated speed, as described above. The direction of rotation is electronically controllable up to half speed; the two directions of rotation require opposite phasing of the AC line voltage applied to S2 prior to synchronization and closing of AC switch 16.
For variable torque applications such as fan and pump drives, the number of turns on the stator S1 of motor 60 in FIG. 2B is doubled, which halves the speed range at full motor excitation to give the speed-torque capability shown in FIG. 5A. Maximum speed with this winding is 1350 RPM, limited by the available voltage from the controller, as shown by a plot 106 in FIG. 5C; synchronous speed is 900 RPM in this example. Excitation weakening in the middle third of the speed range reduces the torque as shown in a plot 102, but provides a minimum of about 50% of full torque in this range as needed for variable torque loads, with a controller rated at about one-third of the full rated motor power. Full torque shown in plot 102 is available in the lower third of the speed range for starting the load and in the upper third of the range where the running load is high. Starting and operation up to about one half synchronous speed, in the region under box 86, are as described above, except that the S1 voltage shown in plot 106 of FIG. 5C is twice that in the configuration used for the plots of FIGS. 4A, 4B, and 4C. Controller output frequency shown in a plot 104 and the voltage shown in a plot 106 are proportional to motor speed up to the half-synchronous speed level. Controller output power is proportional to the product of speed and load, reaching a maximum of 33% of rated motor power at the half-synchronous speed level and rated load, neglecting losses. From half-synchronous speed to synchronous speed, the motor excitation is progressively reduced with increasing speed to maintain near-maximum voltage, as shown in plot 106 and to provide constant power capability equal to about 33% of the motor rating. In this range, controller output power is proportional to motor power output, reaching about 33% of motor rating maximum, neglecting losses. Switching to doubly-fed control with the AC switch closed, as indicated in box 88, at or near synchronous speed, and operation above synchronous speed are as described above.
The AC line supplies power proportional to output torque to stator S2 at all speeds above synchronous speed, once the AC switch is closed. Neglecting losses, this power ranges from zero at no load to about 67% of the full rated motor output power at rated torque. Again neglecting losses, the controller supplies power proportional to the product of torque and the difference between synchronous speed and operating speed to stator S1 at speeds above synchronous speed. This power ranges from zero at synchronous speed, to about 33% of motor rating at maximum speed and rated load. The direction of rotation is electronically controllable up to synchronous speed. The two directions of rotation require opposite phasing of the AC line voltage applied to S2 prior to synchronization and closing of the AC switch.
The machine is operated as a variable-speed constant-frequency generator at a speed range above and below synchronous speed (the speed at which the first stator input frequency is zero). A speed range of xc2x125% synchronous speed can be attained with a control rated at 20% of the generator output power at maximum speed. Other speed ranges require a controller rating proportional to the speed variation from synchronous speed.
The processor-based control of this invention enables machine 40 to generate power into AC power line 14 (co-generation) by operating the machine as a flux vector controlled doubly-fed generator, as shown in FIG. 2A, with processor-based control 50 configured as a three-phase current regulator and AC switch 16 closed. The flux vector control mode continuously maintains stator S1 flux orientation relative to the applied AC power line voltage within the capability of control 50, even for suddenly applied electrical loads, or loads beyond the torque capacity of the prime mover driving the shaft of the machine. Synchronization with the AC line before initiating generation is accomplished by processor-based control 50 at any speed in the operating range, by automatically controlling the S1 excitation to synchronize the generated S2 voltage at taps 18 with AC power line 14, and then closing AC switch 16 to accomplish bumpless connection of the generator to the AC power line. Control 50 then switches to doubly-fed flux vector mode, to control the current generated and the torque over the generating speed range.
FIGS. 6A, 6B, and 6C illustrate a plot 112 of control power, a plot 114 of total power, a plot 116 of stator S1 frequency, and a plot 118 of voltagexe2x80x94all versus speed, for dual 2-pole brushless doubly-fed generator 40 connected to 60 Hz AC power line 14. Stator S1 is wound with four times the number of turns on S2, giving it four times as high a voltage constant and limiting the generating speed range to xc2x125 percent of the synchronous speed (e.g., 1800 RPM). Stator S2, which is connected to the AC line, supplies about 80% of the generated power (shown in a plot 110) provided to the AC line by the machine at maximum speed, and the control supplies about 20% of the output power (as shown in plot 112) from S1. For the same torque load on the prime mover, S2 continues to supply about 80% of the maximum speed power at any lower speed, while the control reduces the power supplied from S1 with speed to zero at synchronous speed and to a negative level below synchronous speed. With the same prime mover torque load applied at a minimum speed of about three quarters synchronous speed, the control absorbs 20% of the maximum speed power (shown in plot 112) from S1. Stator S2 continues to supply about 80% of the power (as shown in plot 110) to the AC line, resulting in a net generated power (shown in a plot 114) of about 60% of that available at the maximum speed of about 1.25 times synchronous speed (i.e., 2250 RPM, in this example).
In accord with the present invention, a processor-based control 72 of FIG. 2C operates machine 70 for standalone power generation in applications without an AC line, by functioning as an inverter. A direct current (DC) bus starting power source 42 provides control power to the inverter and other controls until the generator output in lines 34 is sufficiently great to supply the inverter and control power output from stator S2. The plots of FIGS. 6A, 6B, and 6C and the above discussion of power distribution between the stators are also applicable to standalone power generator operation.
The control can be remotely switched between these two modes of operation to enable co-generation by the machine when an AC line is available, and standalone generator operation when AC line power is unavailable. In accord with the present invention, for multiple installations of such machines, one machine can be operated in the standalone mode to establish an AC line voltage and frequency while the other machines are operated in the co-generation mode.